Standard overhead projectors, which have been used for many years, project images onto a screen from a thin transparent plastic sheet, commonly referred to as a transparency. As shown in FIG. 1, projector 10 includes a base 12 housing a light source, and a lens system 14. A transparency 16 is placed on top of base 12 where light emitted from the light source shines through transparency 16. Lens system 14 directs the image of the information printed on transparency 16 onto screen 18. A disadvantage of a standard overhead projection system is that it cannot interface with a computer. The system is capable of displaying only the image printed on a transparency. Other disadvantages of a standard overhead projection system are that it is too heavy to be easily portable, it produces a great deal of heat, and it consumes a large amount of power.
Other overhead projector systems in use today, however, are capable of interfacing with computers. One such projector system is the BARCO Data 600 manufactured by BARCO Projection Systems and shown as prior art in FIG. 2. Projection system 20 uses three monochrome cathode ray tube (CRT) projection tubes 22, each individually projecting a monochromatic light signal corresponding to the image of a primary color, which images are converged using colored filters and lenses 24 to create a composite color image on screen 26. Projection system 20 can be interfaced to a computer (not shown), thereby allowing the images appearing on the computer's display to also be projected onto screen 26. However, such a system cannot be used to project a image printed on a transparency. Other disadvantages of this type of projection system are that it is not portable, and it is large because of the physical depth required by the CRT technology.
There is no known projection system which is capable of projecting both a computer-generated video image and an image printed on a transparency, either selectively or concurrently. Such a system is highly desirable in order to provide a true multimedia presentation. Other features which may be desired in such a system include portability, reduced power consumption and reduced heat generation.